In oil and gas production and exploration, downhole drilling can be accomplished with a downhole drill through which drilling fluid, conventionally referred to as drilling mud, is pumped. The drilling fluid assists in the drilling process in a number of ways, for example by dislodging and removing drill cuttings, cooling the drill bit, and providing pressure to prevent formation fluids from entering the wellbore.
It has been found that applying a vibrational and/or percussive effect, which can be accomplished through regulation of the drilling fluid flow, can improve the performance of the downhole drill. Examples of downhole drill assemblies providing such an effect are described in Canadian Patent Application No. 2,798,807, having common inventors with the present application, and Canadian Patent No. 2,255,065. In some cases, a vibrational or percussive effect can adversely affect measurement while drilling (MWD) or survey equipment mounted in the drilling string.